When creating animations, how come we start at frame 1, not frame at 0?
It's this way because just about everything else that is indivisible in the world is that way, and for good reason. We start at frame 1 because it's the first frame. Sure 0 could be used, but having 0 as the first frame instead of 1 would be unnecessarily confusing since frame 1 would now not be the first frame and everything would be offset by 1.

It may be easier to understand if you think of a similar situation where you start at one and not zero, such as counting things (which is technically what we are doing here) or book pages.